1. Field of the Invention
The present invention relates to a technology for cleaning a back surface of a substrate such as a semiconductor wafer and a glass substrate (Liquid Crystal Display (LCD) substrate) for LCD.
2. Description of the Related Art
In a fabrication process of a semiconductor device, it is very important to keep, for example, a semiconductor wafer (referred to as just a wafer hereinafter) clean. Therefore, various cleaning processes for cleaning a top surface of the wafer are carried out, when necessary, before or after various processes.
Generally, the top surface of the wafer can be cleaned by placing a brush on the top surface of the wafer that is firmly fixed on, for example, a vacuum chuck or a mechanical chuck, and moving the brush with respect to the wafer while supplying deionized water or the like to the top surface. With this, particles on the top surface are removed.
Such cleaning has come to be necessary for a back surface of the wafer in addition to the top surface on which a circuit is formed. For example, when particles stay on the back surface of the wafer and the wafer is placed on a wafer stage in a stepper, the particles between the back surface and the wafer stage may bend the wafer, leading to a so-called defocusing problem in the photolithography process. Since further reduction in line width is being pursued in the fabrication processing of semiconductor devices, which brings forth new process technologies, such as immersion lithography, double patterning, or the like, defocusing is thought to be a more serious problem. Moreover, since not only the new processes but also new materials to replace conventional materials are emerging along with the line width reduction, the number of processes tends to be increasing, which may create more particles on the back surface of the wafer. Therefore, cleaning the back surface of the wafer has drawn more attention than ever.
By the way, the photolithography process is carried out in a photolithography system composed of a photoresist coater/developer in which a photoresist is spun on the top surface of the wafer and the photoresist is developed, and a subsequent exposure apparatus in which the developed photoresist is exposed so as to produce a predetermined pattern in the photoresist. Since the wafer transferred inside the system is usually kept face-up, the wafer needs to be reversed up-side-down by a so-called wafer reverser in order to clean the back surface of the wafer when a cleaning tool employing the brush to be pressed on the wafer from above is used. If such a wafer reverser is placed in the photolithography system, the system has to have a space for the reverser itself and a space for reversing the wafer, which inevitably increases the size of the photolithography system. In addition, even if the brush for cleaning the back surface of the wafer is disposed below the wafer in order to eliminate the need for the wafer reverser, the brush cannot entirely clean the back surface since the wafer is usually supported from below by the wafer chuck or the like, which disturbs the cleaning.
In order to address such problems, the Patent Document 1 below has proposed a substrate cleaning apparatus including a newly designed spin chuck for rotatably holing the wafer. This spin chuck is shaped into a hollow cylinder having substantially the same diameter as the diameter of the wafer. The spin chuck has inside the cylinder the brush and a nozzle for ejecting cleaning liquid toward the back surface of the wafer. With this configuration, the wafer is transferred face-up to the hollow cylinder, and held at the wafer edge by a mechanical chuck or the like that is located at the top end portion of the cylinder. Next, the brush inside the cylinder is raised so as to contact the back surface of the wafer, and the cleaning liquid is discharged from the nozzle. Then, the wafer is rotated by the spin chuck and the brush is shifted with respect to the wafer, which enables cleaning the entire area of the back surface of the wafer without using the reverser.
Patent Document 1: Japanese Patent Publication No. 3377414 (paragraphs 0036 through 0040, FIG. 3)
However, since the wafer is held at its edge by the mechanical chuck or the like on the top end portion of the hollow cylinder in the substrate cleaning apparatus disclosed in Patent Document 1, the possibility of wafer edge damage cannot be completely denied. In addition, the following problem may be caused. When immersion lithography is carried out, the edge of the resist layer forming on the wafer extends generally to bevel and vertical portions (or round portion) of the wafer edge in order to avoid the edge of the resist layer being wetted with the deionized wafer used in the immersion lithography. This is because the resist layer may be exfoliated if the edge of the resist layer becomes wetted with the water. In this situation, if the wafer is held at its edge by the mechanical chuck, the resist layer may be damaged, thereby generating particles and causing possible exfoliation of the resist layer.
The present invention has been made in view of the above, and is directed to a substrate cleaning apparatus and a method of cleaning a substrate which enable cleaning the back surface of the wafer without reversing the substrate and can avoid the circumferential portion of the wafer being damaged.